This invention concerns improvements in or relating to optical filtering apparatus, and relates more particularly to such apparatus employing a holographic wavelength selective reflector.
Holographic wavelength selective reflectors can have advantages over other types of spectral optical filters by way of improved performance in terms of efficiency and a well defined spectral passband (i.e. the waveband selectively reflected by the hologram). However, there remain requirements for even better defined, and possibly narrower, spectral passbands. There is also a continuing requirement for reduction of unwanted background arising from spectral sideband reflections of the hologram and/or surface reflections at the surfaces of the substrate carrying the hologram coating. The latter can be reduced by application of an appropriate anti-reflection coating to the relevant surface but this tends to be a fairly expensive operation.